(a) Field of the Invention
The present invention relates to an inverter driving device and method. More specifically, the present invention relates to an inverter driving device and method for frequency and duty control.
(b) Description of the Related Art
In general, a backlight used as a light source of an LCD uses a fluorescent lamp to configure a surface light source with uniform brightness. A CCFL (cold cathode fluorescent lamp) of a small size and enabling high-luminance light emission is generally used for the fluorescent lamp.
Conventional inverter drivers have a controller for controlling the current flowing to the CCFL so as to uniformly maintain the brightness of the backlight.
When an input voltage of the inverter driver or a load at the CCFL is varied, the current flowing to the CCFL is varied to change the brightness of the backlight. Therefore, the conventional inverter drivers include a control signal supply for sensing the input voltage and a voltage corresponding to the current flowing to the CCFL to uniformly maintain the current flowing to the CCFL.
The conventional inverter driver uses an output voltage of the control signal supply to control the current flowing to the CCFL by using one of a duty control method and a frequency control method.
The inverter driver according to the conventional duty control method controls switching states of first and second switches coupled in series between the input voltage and the ground voltage to reduce the duty when the input voltage rises or the current flowing to the CCFL is high, and output a duty-increased pulse signal when the input voltage falls or the current flowing to the CCFL is low. A transformer converts the pulse signals to control the current flowing to the CCFL.
However, in the inverter driver according to the conventional duty control method, the current waveform of the CCFL is steeply varied when the duty greatly reduces or increases. Accordingly, the brightness of the CCFL becomes unstable, interference occurs in the adjacent circuit because of many harmonics thereof, and the lifetime of the CCFL shortens.
The inverter driver according to the conventional frequency control method performs control by varying the operation frequency of the current at the CCFL. The inverter driver uses a variable resistor coupled to an oscillator to modify the current flowing to a capacitor coupled to the oscillator, thereby varying the operation frequency of the current of the CCFL.
In this instance, since the conventional inverter driver directly connects the current generated by comparing the reference voltage and the voltage at the load coupled to the CCFL to the variable resistor, it is difficult to control the variation range of the operation frequency. Also, when the CCFL dims, the maximum frequency may rise to 200 KHz which causes an EMI problem, a switching loss problem, and a problem of digressing from the CCFL operation frequency range.